This invention relates to a method and apparatus for combining video signals.
A typical television broadcast is formed by a sequence of static video effects, each composed of a background and, optionally, at least one key. The key is an area of key video that appears to be in front of the background and is limited spatially by an associated key control signal. In the context of a news broadcast, the background of the current, or on-air, effect might be a studio backdrop and the foreground of the on-air effect might be a newscaster, while the background of the next effect in the sequence might be a scene of a sports events and the foreground of the next effect a sports announcer. Alternatively, the background of the on-air effect might be a scene of a sports event and the foreground of the on-air effect a sports announcer, while the next effect is the scene of the sports event without the sports announcer. Depending on the key control signal, the area of the key can change, for example when the sports announcer gestures or a weather forecaster points to a region of a weather map background, but nevertheless the effect is considered to be a static effect.
A dynamic effect, or transition, occurs when there is a change from one static effect to another static effect. A change in the area occupied by a key is not considered to be a transition. A mix/effects (M/E) device is used to provide a transition that is not visually disturbing to the viewer.
One method of providing a transition from the on-air effect to the next effect is to use a wipe border matte effect. The wipe border matte effect will be described with reference to FIG. 1, which illustrates a typical serial architecture for an M/E device.
The M/E device shown in FIG. 1 comprises six video mixers 9-14 connected in series. Each mixer receives a mix control signal at its X input and full-field video signals at its Y and Z inputs and provides an output signal represented by the equation: EQU Output=XY+(1-X)Z
The mix control signal can range in value between 0, or fully off, and 1, or fully on, and therefore the output signal can be all Y, all Z, or a linear mix of Y and Z. The mix control signal can vary as a function of position in the video field and/or as a function of time, from frame to frame.
Mixer 9 receives a border matte video signal at its Z input, and the outputs of mixers 9-13 are connected to the Z inputs of mixers 10-14 respectively. The desired output signal is provided at the output of mixer 14.
Mixers 9 and 10 receive at their Y inputs respective video signals of program A and program B, which are the backgrounds of the on-air effect and the next effect respectively. Mixers 11-14 receive at their Y inputs respective key video signals, designated key 1 video-key 4 video. Key control signals, designated key 1 control-key 4 control, are associated with key 1 video-key 4 video respectively and will be discussed further below. Each key control signal is similar to a video signal, and is used to define an area of the video field.
The mix control signals received by mixers 9 and 10 at their X inputs are referred to as transition control signals and are designated transition control signal T1 and transition control signal T2 respectively. The transition control signals T1 and T2 are generated by a transition control signal generator 16 which functions in response to signals provided by an operator interface 18. The mix control signals received by mixers 11-14 are provided by respective multipliers 21-24, each of which receives a key control signal at its X input. The Y inputs of multipliers 21-24 are connected to respective selector switches 31-34, each of which is able to select logical 0, logical 1, T1 or T2. The output signal of each multiplier is represented by the equation: EQU Output=XY
It will be apparent from FIG. 1 that the selector switches 31-34 can be set so that if the transition control signals T1 and T2 were set to 1 and 0 respectively for the entire field, an effect could be composed of program A background with key 1 video within the area defined by key 1 control. If, on the other hand, the transition control signals T1 and T2 were set to 0 and 1 respectively for the entire field, an effect composed of program B background with key 2 video within the area defined by key 2 control could be provided. If, as shown schematically in FIG. 1, the transition control signal T1 were 1 for a region at the right of the field and the transition control signal T2 were 1 for a region at the left of the field, an effect could be composed in which program A plus key 1 is shown at the right of the field, program B plus key 2 is shown at the left of the field, and the border matte appears as a vertical stripe down the center of the field. The transition control signals T1 and T2 can be varied dynamically in response to control signals provided by the operator interface 18, such that the size of the region for which the transition control signal T1 is 1 becomes smaller and the size of the region for which the signal T2 is 1 becomes larger. Thus, the vertical stripe sweeps across the field from left to right, providing a wipe from an on-air effect composed of program A plus key 1 to a next effect composed of program B plus key 2.
It will be appreciated that in the case of the example that has just been described, mixers 13 and 14 are not used and accordingly selector switches 33 and 34 are each set to the zero input.
The M/E device shown in FIG. 1 is subject to certain disadvantages. For example, during a mix transition or a soft wipe, when the mix control signal for a given key mixer is between 0 and 1, so that the associated key video should be present at the output of the mixer, the key video is partially transparent and a lower priority key video and/or background video might be visible through the higher priority key video. Also, in the ME device shown in FIG. 1, the background of the output effect is created by mixers 9 and 10 and the keys are added by mixers 11-14 after the background has been composed, and therefore the ME device is unable to provide a preview output of the next effect.
FIG. 2 shows a parallel M/E device in which each component of the desired output effect is multiplied by a control signal that limits the component to the desired area of the video field. The desired output effect is composed in the manner of a patchwork. A disadvantage of the M/E device shown in FIG. 2 is that the processing of the control signals can be quite complicated, and therefore the circuitry for generating the control signals is complex. As in the case of FIG. 1, a preview output of the next effect cannot be provided.